Electric fuses

ABSTRACT

An electric fuse comprises a metal ribbon provided with indentations or openings to form zones of narrowed cross-section serving as fuse elements, the adjacent portions of the ribbon serving as heat sinks and cooling surfaces. According to the invention, one or more insulating stiffening members is/are attached to the ribbon to improve mechanical strength in the weakened zones without affecting arc formation. In a preferred method of manufacturing, the final formation of the fuse elements takes place after the stiffening member has been attached.

United States Patent Lerstrup 1 Sept. 12, 1972 [541 ELECTRIC FUSESFOREIGN PATENTS OR APPLICATIONS Inventor: Karl Lerslrup, p g, 954,5134/1964 Great Britain ..337/290 De 696,232 10/ 1930 France ..337/293 [73]Assignee: Aktleselskabet Pour. Knudsen Nor- Primary Examiner Bemard AGilheany disk Elektncitets Selskab, Copenhagen D enmark AssistantExammerDew1tt M. Morgan Attorney-Watson, Cole, Gnndle & Watson [22]Filed: Aug. 17, 1970 21 Appl.- No.: 64,347 [57] ABSTRACT An electricfuse comprises a metal ribbon provided with indentations or openings toform zones of nar- [52] US. Cl. ..29/623, 337/166, 337/297 h d 51 int.Cl....H01h 69/02, H01h 85/10, HOlh 85/14 cmsis'sectm sefvmg as Felements. e a [58] Field of Search 337/158 290 291 292 295 acentportions of the ribbon servmg as heat sinks and 337/231 293 coolingsurfaces. According to the invention, one or 7 more insulatingstiffening members is/are attached to the ribbon to improve mechanicalstrength in the [56] References Cited weakened zones without affectingarc formation. In a UNITED STATES PATENTS preferred method ofmanufacturing, the final formation of the fuse elements takes placeafter the stifien- 3,152,233 10/ 1964 Kozacka ..337/ 158 X ing memberhas been attached 3,465,275 9/1969 Swain ..337/295 X 1,601,726 10/1926Eustice ..337/295 1 Claim, 6 Drawing Figures BACKGROUND OF THEINVENTION.

This invention relates to an electric fuse, and more particularly to thefuse element'or elements of such a fuse. The invention will be describedas applied to a sand filledfuse with current limiting action, but theinvention is not limited to such fuses.

For such fuses it has been customary to use one or more bands or ribbonsof metal from which the fuse element or elements are formed by punchingvariously shaped holes or indentations'in the ribbon. In some cases itis intended that the major part of the ribbon shall melt during theoperation of the fuse, that is, the ribbon itself may be considered thefuse element, but in other cases it is intended that only specific partsof the ribbon shall melt, while other parts remain intact duringoperation. In this case only the parts intended to melt are said to formthe actual fuse elements, while the remaining parts of the ribbon formthe terminals of the fuse elements and serve as heat sinks and coolingsurfaces. The present inventionrelates in particular to this lattertype.

To achieve this goal of having part or parts of the ribbon melt, whileother parts remain solid during the entire fuse operation it isnecessary that the cross-section through which the electric currentflows is significantly smaller in the actual fuse element or elementsthan in the parts of the ribbon which serve as heat sinks and coolingsurfaces. This difference in cross-section causes all mechanicalstresses to be concentrated on the smaller cross-sections, and when thedifference is large, the risk of overstressing the fuse elements duringhandling is likewise increased, and this forms a practical limit to theratio of the largest to the smallest crosssection.

It is the object of the invention to provide a mechanical design of suchfuse elements that permits a large ratio of cross-section withoutsubjecting the actual fuse element to undue mechanical stress. It is afurther object to provide for such sequence of manufacture that thefirst object can be achieved without necessitating extreme care duringthe process of manufacture.

SUMMARY OF THE INVENTION.

According to the invention, there is provided an electric fuse, havingfuse elements integral with heat sinks and cooling surfaces, formed bythe removal of parts of a metal ribbon, and provided with one or moreinsulating stiffening members fixed to the ribbon.

BRIEF DESCRIPTION OF THE DRAWING.

FIGS. 1 and 2 are diagrammatic figures showing certain aspects of thepresent state of the art.

FIG. 3 illustrates an aspect of the known art which forms an importantpart of the invention.

FIG. 4 serves to explain the basic aspects of the invention;

FIG. 5 illustrates a particular practical embodiment of the invention;and

FIG. 5a is a section taken along lines 5-5 as indicated in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS.

The known way of producing a fuse element of the kind to which theinvention is related is shown in FIG. 1, where 1 is a band or ribbon ofmetal from which a fuse element is formed, 2 and 2' are indentations cutfrom the sides of the ribbon, leaving only a narrow part of the metal toform the actual fuse element 4, while the remaining partsof the ribbon lin the vicinity of the fuse element 4 form heat sinks 5 and 5 that atthe same time provide or serve as cooling surfaces.

FIG. 2 shows how several such fuse elements can in known manner beformed in the same ribbon in such a manner that the melting of the fuseelements results in the formation of several arcs connected in series,the reference characters being the same as in FIG. 1.

If the ratio of the width of the basic ribbon 1 to the width of theindividual fuse elements 4 becomes extremely high, as is desirable forcertain kinds of fuses, for instance in the order of 10 to 1 or more,the fuses according to FIGS. 1 and 2 become mechanically weak and thereis a grave risk that the fuse elements have been mechanicallypre-damaged before the fuse is placed in service, and this seriouslyaffects its electrical characteristics. The risk is the greater, thegreater the ratio of the cross-sections, and the more fuse elementsthere are in series according to FIG. 2.

FIG. 3 illustrates a first step towards the alleviation of thisweakness. This figure is directly comparable with FIG. 1 and again 1 isthe ribbon from which the fuse elements are formed and 5 and 5' are theheat sinks and cooling surfaces, but in addition to the two indentations2 and 2', which are not quite so deep, an opening 2" has been addedbetween the two indentations. In this manner the original fuse element 4of FIG. 1 has been replaced by two fuse elements 4' and 4" in parallelin FIG. 3. If now each of the two fuse elements 4' and 4" have half thewidth of the fuse element 4 of FIG. 1, then the two designs of FIG. 1and FIG. 3 are equivalent with respect to reduction of cross-section,but it is clearly seen that the design of FIG. 3 has considerablemechanical advantages over that of FIG. 1.

In FIG. 1 any mechanical moment acting in the plane of the ribbon andtending to move the part 5 with respect to the part 5' will easily bendthe narrow neck 4, while the two necks 4' and 4" of FIG. 3 will resist aconsiderably larger amount without permanent defor mation. However, thedesign illustrated in FIG. 3 also has to be regarded as belonging to theprior art, even if the motivation for its use has not been founded onmechanical considerations.

While the design exemplified in FIG. 3 represents an importantimprovement over that of FIG. 1 for any moment having a component in theplane of the ribbon, it is no better than that of FIG. 1 with respect toa moment tending to bend the ribbon out of the plane of the paper.

FIG. 4 shows a ribbon with fuse elements according to the invention. Theselected example for illustration is a ribbon with several fuse elementsin series and with the minimum of two sets of fuse elements in parallel,but the invention is by no means limited to this embodiment. The fuseelements have been formed on the principles illustrated in FIG. 3, butin addition a stiff insulating strip 6 is fixed to the ribbon in such amanner that it adds stiffness against bending of the fuse element in thedirection out of the plane of the paper. It is noted that thisstiffening member must not necessarily provide so much stiffness that itprevents bending of the ribbon, but only so much and of such a characterthat it prevents excessive bending at the location of the narrow necksforming the fuse elements.

An essential feature of this stiffening strip is that it is so locatedon the ribbon that it does not interfere with the formation of arcs whenthe fuse elements melt. This can be achieved, as illustrated in FIG. 4,by a simple strip that is narrow compared to the width of the openings 2in' the ribbon so that the necessary distance from the fuse elements tothe strip is secured, but it is also possible to use a wider stripprovided it has indentations of sufficient width and depth to providethe desired distance from the fuse elements, or, if it is wide enough,is provided with openings at the-location of the fuse elements.

It is within the scope of the invention to fix stiffening strips orplates to both sides of the ribbon, and it is also within the scope ofthe invention to let the insulating stiffening member cover the fuseelements partly or completely, but in that case only stiffening on oneside can be used.

The stiffening member can be fixed in relation to the ribbon of metal byany suitable means. Examples of fixing means that have been foundsuitable are gluing, riveting or deformation of part of the ribbon toclamp the strip, but any practical means of fastening is consideredwithin the scope of the invention.

FIG. and FIG. 5a illustrate a specific method of fixing the stiffeningstrip to the metal ribbon. FIG. 5 repeats part of FIG. 4 with the fixingmeans added and FIG. 5a further illustrates this by showing across-section of the ribbon and the stiffening strip, but for the sakeof clarity the dimensions are out of proportion. Again 1 is'the metalribbon, 4' and 4" the fuse elements located on either side of thestiffening strip 6, which is fastened to the ribbon by means of wirestaples 7. In the illustration, the staples surround the stiffeningmember, but pierces the metal ribbon. However, such details areimmaterial for the invention.

In the examples used to illustrate the invention a design according toFIG. 3 with two parallel fuse elements has formed the basis. However, tothe man skilled in the art it will be obvious that the invention alsocovers designs having more parallel fuse elements and a singlestiffening strip, as well as the use of several stiffening strips orstiffening members on the same ribbon, including the use of a singlefuse element, provided the stiffening strip and its fixation to theribbon are such that it can transmit the necessary forces without unduestress to the fuse element.

However, in case the ratio of the cross-sections is extremely high therestill remains the risk of mechanical damage to the fuse elements duringthe interim between the shaping of the ribbon and the fixing of thestiffening member and in such cases a special manufacturing proceduremay be necessary and forms part of the invention. On principle it wouldbe desirable to fix the stiffening member before the ribbon is punchedto give it the desired shape, but obviously this is impossible seeingthat in the embodiments illustrated the stiffening mem ers over rgart ofthe ptpenings or indentatrons orme int e rib on. This d1 iculty isovercome by forming in a first manufacturing step only the openings tobe covered by the stiffening member, fixing the stiffening member in asecond step, and forming in a final step the actual fuse elements whichin this embodiment cannot be covered by the stiffening member. Anothersolution is to make the stiffening member so wide in relation to thewidth of the metal ribbon that it is possible to have openings in thestiffening member wider than the width of the ribbon.

Iclaim: 1. A method of making a fuse element strip for a sand-filledfuse, comprising the steps of:

punching a row of spaced holes longitudinally along a metal ribbonwherein the spacing of said holes provides sufficient mechanicalstrength of said metal ribbon for subsequent processing; attaching atleast one insulating strip having a width less than the transversedimension of said holes to at least one surface of said metal ribbonwith the edges of said insulating strip spaced from the outer transverseportions of the edges of said holes; and punching additional materialportions from said metal ribbon to form narrow necked fuse elementportions spaced from said insulating strip.

1. A method of making a fuse element strip for a sand-filled fuse,comprising the steps of: punching a row of spaced holes longitudinallyalong a metal ribbon wherein the spacing of said holes providessufficient mechanical strength of said metal ribbon for subsequentprocessing; attaching at least one insulating strip having a width lessthan the transverse dimension of said holes to at least one surface ofsaid metal ribBon with the edges of said insulating strip spaced fromthe outer transverse portions of the edges of said holes; and punchingadditional material portions from said metal ribbon to form narrownecked fuse element portions spaced from said insulating strip.